Photodetectors comprised of a compound semiconductor material system have sensitivity in the wavelength regions that silicon detectors do not cover have been widely used particularly in the infrared region as the most sensitive photodetector. Compound semiconductor photodiode arrays are in broad demand such as sensors for infrared spectroscopy, monitors for optical communications with wavelength division multiplexing, infrared image sensors and various industrial measurements. Compound semiconductor arrays mostly used in many ways are photodiode arrays having the light receiving layer of the InGaAs layer formed by epitaxial growth on the InP substrate. For the detection of longer wavelength of the near-mid infrared region, photodiode arrays comprised of the InSb system, the InAs system and the HgCdTe system have been used.
The dark current which determines the detection limit of semiconductor photodetector is originated both in the internal semiconductor material and in the semiconductor surface. In order to suppress the dark current due to the internal semiconductor material, it is effective to decrease the carrier generation by thermal excitation in the semiconductor layers other than the photosensitive layer used to absorb photons. In addition, the carrier generation rate due to thermal excitation is proportional to the square of the intrinsic carrier concentration in general, so that it is effective to place the photosensitive layer sandwiched by the semiconductor layers with large energy band gaps, that is, with lower intrinsic carrier concentrations in order to reduce the dark current. In addition, it is desirable to locate the metallurgical PN junction exposed to the surface of the semiconductor layers with a wide energy band gap in order to reduce the dark current originated in the semiconductor surface region.
For example, in the planar photodiode (PD) based on the InGaAs/InP system described in the document 1, the surface associated with high crystal defect density is covered with the InP layer that has a relatively large energy band gap, and the PN junction formed in the InGaAs photosensitive layer having a smaller band gap is not exposed to the surface. In the process to produce a planar type PD, a PN junction is formed by diffusing impurities from the surface selectively, and arrays are obtained by arranging PD elements in one-dimensional or two-dimensional array patterns.
This kind of structures are simple in fabrication process but have disadvantage that the cross-talk between the elements is high. This crosstalk is caused by the fact that carriers generated in the photosensitive layer flow easily into adjacent elements by carrier diffusion and induce output signal in adjacent elements. As a method to improve the crosstalk between the elements is to provide a light shielding mask between the elements as shown in the document 1. This method is effective to prevent the photo-excited current in the gap between the elements but it cannot prevent carriers generated just under the element from flowing into adjacent elements completely and moreover, the light shielding mask is invalid when arrays are configured to receive incident light from the substrate side. And also, to form a light-shielding mask on arrays with a narrow pitch less than 10 few micrometers induces problems in the production yield.
As a measure to improve the shortcomings of such planar type PD arrays, a method to isolate elements in a mesa type has been taken as shown in the documents 2, 3, 4, and 5. All these examples include the InGaAs photosensitive layer and the InP window layer formed on the N type InP substrate, and each element is isolated in mesa structures. These structures bring in some advantages that cross talk is substantially improved as compared with that of a planar structure, and a light-shielding mask is not necessary.
However, the metallurgical PN junction is exposed to the surface in the mesa structure formed in a compound semiconductor so that a surface leak current is significantly increased. The large surface leak current causes a noise and deteriorates the minimum light receiving sensitivity. The surface of the mesa has been passivated by a dielectric film such as silicon nitride film. In addition, the photosensitive layer of InGaAs is usually made as an I (intrinsic) layer with the carrier concentration as low as to the order of 1014 cm−3 so that electrons tend to accumulate at the surface by defects and pollution at the interface between the surface dielectric film and the semiconductor surface, or by ions contained in the dielectric film, or by polarization of the dielectric film. Such accumulated electrons induce unstable electrical phenomenon as to invert the I type mesa surface area and deteriorate PD characteristics with time.
The mesa type PD array with a PIN structure is proposed in the document 6, which includes the InGaAs photosensitive layer and the N-type InP formed on the P-type InP layer for the purpose of obtaining a fast response when light incidents on the back surface. The PN junction of the photosensitive layer shown in the document 6 is protected either by the insulating film formed on the semiconductor side surface, or by the buried layer with high resistance but the crystal surface or the re-growth interface is in a depletion state so that the problem of surface leakage originated in crystal defects in the vicinity of the surface still remains.
The documents 7 and 8 describe PDs having a mesa structure where the PN junction with a narrow forbidden gap is not exposed to the surface. The dark current can be suppressed by heavily doped impurities with a diffusion method at the surface region of the PN junction exposed to the end face of the mesa structure to extinguish the depletion layer near the surface. However, in both, when PD elements are arranged in an array, it requires to form a PNP structure after removing the photosensitive layer and exposing the buffer layer with a large band gap on the surface to cut off the current in the mesa bottom layer and in order to ensure electrical isolation of each element. In these structures, the front of the impurity diffused layer requires to be kept inside in a buffer layer with the large energy band gap, and the fabrication process was complicated as acceptable control range of process conditions is narrow. In addition, in the case of the document 7, in order to form a PN junction of the photosensitive layer by impurity diffusion, there is a problem that the junction is formed in a relatively deep position, which reduces the sensitivity in the short wavelength side.